1. Field of the Invention
The present invention relates to an optical modulator, and in particular to an optical modulator having an electric waveguide and an optical waveguide.
Recent optical communication technology has been remarkably developed, leading a trunk information communication network to a high-speed optical communication, so that optical fibers are going to be brought into households. With this development, it has become more and more important to enhance the speed of an optical modulator which puts information on a lightwave, presenting one of the basic communication technologies for transmitting a large amount of information at a high speed.
2. Description of the Related Art
As a prior art optical modulator, a Mach-Zehnder type optical modulator of LiNbO3 (Lithium Niobate, hereinafter abbreviated as LN), for example, is an optical intensity modulator having a good transmission characteristic, by the combination of an LN optical phase modulator and a Mach-Zehnder type interferometer. It is used for many transmitters of a high optical transmission speed such as 2.4 GHz, 10 GHz, and 40 GHz.
FIG. 12 shows an arrangement of an optical phase modulator 100xe2x80x2, which is composed of an electric waveguide 20 (generally referred to as electrode) and an optical waveguide 10. A modulating signal 71 from a modulating signal generator 40 is inputted to the electric waveguide 20 as a modulating signal 71a through a driver 50.
The electric waveguide 20 converts the inputted modulating signal 71a into an acting amount (modulating amount) 80 to be provided to the optical waveguide 10. The acting amount 80 is for providing a modulation to a lightwave 81 which propagates through the optical waveguide 10.
In case of an optical phase modulation made by an electro-optical effect for example, the acting amount 80 is proportional to the product of a modulation voltage V (electric field E) and its acting interval L.
Since there is a resistance caused by a skin effect in the metal of the electric waveguide 20 in a modulator such as a traveling wave type optical phase modulator, a frequency characteristic of f1/2 arises as the frequency of the modulating signal increases, thereby narrowing a bandwidth (see characteristic curve A in FIG. 5A). Accordingly, the acting amount 80 of the travelling wave type optical phase modulator has an attenuation in a high frequency area caused by the skin effect (see characteristic curve B in FIG. 5A).
Also, the acting amount 80 of a concentrated constant type optical phase modulator has an attenuation in the high frequency area. This attenuation is determined by an impedance-matching resistance R (not shown) between the electric waveguide (electrode) 20 and the driver 50, and by the frequency characteristic determined by a capacitance C (not shown) and stray capacitances of the electric waveguide 20. It is to be noted that this frequency characteristic is not shown.
In either optical modulator, an intersymbol interference increases as the attenuation increases, so that an optical waveform deteriorates.
It is accordingly an object of the present invention to provide an optical modulator having an electric waveguide and an optical waveguide which suppresses an attenuation in a high frequency band of an acting amount inputted to the optical waveguide.
In order to achieve the above-mentioned object, an optical modulator according to the present invention comprises: an optical waveguide for propagating a lightwave, an electric waveguide for providing an acting amount, to the optical waveguide, for modulating the lightwave by a modulating signal, and a filter for converting the modulating signal into a signal approximating a frequency characteristic of the acting amount to be provided to the electric waveguide.
FIG. 1 shows a principle (1) of an optical modulator 100 according to the present invention. A modulating signal 71 outputted from a modulating signal generator 40 serves to modulate a lightwave 81. An electric waveguide 20 provides, to an optical waveguide 10, an acting amount 80 obtained by converting a modulating signal 72 based on a control method of an optical modulator, e.g. a control method of an electro-optical effect or the like.
The acting amount 80 has a frequency characteristic caused, for example, by stray capacitances of the electric waveguide 20 itself in a concentrated constant type optical phase modulator, and by a skin effect of the electric waveguide 20 itself in a traveling wave type optical phase modulator as will be described later.
Accordingly, when the modulating signal 72 same as the modulating signal 71 is inputted to the electric waveguide 20, the lightwave 81 which propagates through the optical waveguide 10 becomes a lightwave 82 modulated by the acting amount 80 having the frequency characteristic.
In the present invention, a filter 30 converts the modulating signal 71 into the modulating signal 72 for equalizing the frequency characteristic (not a frequency characteristic of electric waveguide itself of the acting amount 80 to be inputted to the electric waveguide 20. Thus, the frequency characteristic of the acting amount 80 is approximated to be almost flat in all of the areas from the low frequency area to the high frequency area, enabling the optical modulator 100 to perform an optical modulation independent of the frequency of the modulating signal 71.
It is to be noted that since FIG. 1 is a schematic diagram, the driver 50 shown in FIG. 12 is omitted.
Also, in the present invention according to the above-mentioned invention, the frequency characteristic of the acting amount may comprise a characteristic caused by a skin effect in the electric waveguide.
Namely, as mentioned above, the resistance of the electric waveguide 20 has the frequency characteristic of f1/2 by the skin effect. Accordingly, the acting amount 80 has the frequency characteristic caused by the frequency characteristic of the resistance. The filter 30 approximates this frequency characteristic of the acting amount 80.
FIGS. 2A and 2B show a principle of approximating the frequency characteristic of the acting amount 80 caused by e.g. the skin effect. FIG. 2B shows a model of the electric waveguide 20, in which a modulating source 40, its internal resistor 42, the electric waveguide 20 having a skin effect resistance, and a terminal resistor 44 are connected in cascade. The values of the resistors 42, 44, and of an impedance Z of the electric waveguide 20 are R0.
FIG. 2A shows a distribution of a voltage v of the electric waveguide 20. The position of an acting interval where the electric waveguide 20 provides the acting amount 80 to the optical waveguide 10 is indicated by a length (distance) x normalized by a length L of the acting interval. Accordingly, x=0 at an input end of the electric waveguide 20, and x=1 at the terminating end. Since the voltage v is a function of the distance x and a frequency f of the modulating signal, it is expressed by the following equation (1):
v=v(f, x)xe2x80x83xe2x80x83Eq.(1) 
Accordingly, in the presence of the skin effect, a transfer function (S21 parameter) indicating the relationship between the voltage v(f, 0) of the input end and the voltage v(f, 1) of the terminating end is expressed by the following equation (2):                               20          ⁢                      log            10                    ⁢                                    v              ⁡                              (                                  f                  ,                  1                                )                                                    v              ⁡                              (                                  f                  ,                  0                                )                                                    =                              -            α                    ⁢                      f                                              Eq.  (2)            
where xcex1 is a constant.
If the voltage v(f, x) is normalized by v(f, 0)=1, Eq.(2) assumes the following equation (3), and accordingly, an output voltage v(f, 1) is expressed by the following equation (4):
20 log10 v(f, 1)=xe2x88x92xcex1{square root over (f)}xe2x80x83xe2x80x83Eq.(3)                               v          ⁡                      (                          f              ,              1                        )                          =                  10                      -                                          α                ⁢                                  f                                            20                                                          Eq.  (4)            
The frequency characteristic of Eq.(4) corresponds to the attenuated amount curve A in FIG. 5A as will be described later.
Since being distributed in the electric waveguide 20 is attenuated exponentially with a function for the distance x from the input end, the voltage v(f, x) is expressed by the following equation (5):
v(f, x)=exe2x88x92xcex2(f)xc2x7xxe2x80x83xe2x80x83Eq.(5) 
where xcex2 is a coefficient depending on the frequency f.
Since the voltage v(f, 1) of the terminating end obtained by substituting x=1 into Eq.(5) coincides with Eq.(4), the following equation (6) is given.                               ⅇ                      -                          β              ⁡                              (                f                )                                                    =                  10                      -                                          α                ⁢                                  f                                            20                                                          Eq.  (6)            
Accordingly, xcex2(f) can be expressed by the following equation (7):                                           -                          β              ⁡                              (                f                )                                              =                                                    log                e                            ⁢                              10                                  -                                                            α                      ⁢                                              f                                                              20                                                                        =                                                            -                                                            α                      ⁢                                              f                                                              20                                                  ·                                  log                  e                                            ⁢              10                                      ⁢                  
                ⁢                              β            ⁡                          (              f              )                                =                                                    α                ⁢                                  f                                            20                        ⁢                          log              e                        ⁢            10                                              Eq.  (7)            
If Eq.(7) is substituted into Eq.(5), the voltage v(f, x) can be expressed by the following equation (8):                               v          ⁡                      (                          f              ,              x                        )                          =                  ⅇ                                    -                              (                                                                            α                      ⁢                                              f                                                              20                                    ⁢                                      log                    e                                    ⁢                  10                                )                                      ·            X                                              Eq.  (8)            
The curve of Eq.(8) is shown in FIG. 2A.
It is supposed that the acting amount 80 which performs a phase modulation to the lightwave 81 (see FIG. 1) is a function q(f) of a modulating frequency f. Also, supposing that the phase modulation is performed by the electro-optical effect, the acting amount q(f) assumes the product of the voltage v and the distance x microscopically. Accordingly, the following equation (9) is given.                               q          ⁡                      (            f            )                          =                              ∫            0            1                    ⁢                                    v              ⁡                              (                                  f                  ,                  x                                )                                      ⁢                          xe2x80x83                        ⁢                          ⅆ              x                                                          Eq        .                  xe2x80x83                ⁢                  (          9          )                    
By substituting Eq.(8) into Eq.(9) to obtain the integral value, the acting amount q(f) can be expressed by the following equation (10):                               q          ⁡                      (            f            )                          =                              20                          α              ⁢                                                f                                ·                                  log                  e                                            ⁢              10                                ⁢                      (                          1              -                              10                                  -                                                            α                      ⁢                                              f                                                              20                                                                        )                                              Eq.  (10)            
Namely, the filter 30 may inversely compensate (equalize) the frequency characteristic of the acting amount q(f) in Eq.(10).
Also in the present invention according to the above-mentioned invention, the electric waveguide may perform an optical phase modulation of the lightwave by an electro-optical effect.
Also in the present invention according to the above-mentioned invention, an interval length in which the electro-optical effect between the electric waveguide and the optical waveguide acts, and a voltage value provided to the electric waveguide may be mutually determined.
As mentioned above, the frequency characteristic occurs by the resistance caused by the skin effect of the electric waveguide 20, which narrows the bandwidth. As measures against it, the prior art optical modulator 100 had the length L of the electric waveguide 20 shortened, and the influence of the skin effect lessened to secure the band.
However, in case the length of the electric waveguide 20 is shortened, a required voltage Vxcfx80 (usually referred to as half-wave voltage) becomes high, since the phase modulation of the lightwave 81 passing through the optical waveguide 10 is proportional to voltage Vxc3x97waveguide length L.
For this reason, as the frequency of the modulating signal increases, the driver 50 (see FIG. 12) which drives the optical modulator 100 is required to have a performance of a high output voltage and a wide band, which leads to a great difficulty in manufacturing.
FIG. 3 shows a relationship between the distance x and the voltage v(f, x) in the electric waveguide 20 shown in FIG. 2A. As mentioned above, the acting amount q(f) provided to the optical waveguide 10 by the electric waveguide 20 is represented by the integral of the acting voltage v in the acting interval (0, 1).
In the optical modulator of the present invention, the frequency characteristic of the acting amount q(f) caused by the skin effect is approximated by the filter. Accordingly, as shown in FIG. 3, the modulating signal 72 added to the electric waveguide 20 is set to a low voltage v1 (v1 less than 1), and an acting interval (0, x1) satisfying the acting amount q1(v1=q(f) can be determined. A normalized x1 at this time becomes longer than the length xe2x80x9c1xe2x80x9d.
Thus, lowering the voltage v which drives the electric waveguide 20 facilitates the design of the driver 50 in the electric waveguide 20.
It is to be noted that in the presence of a limitation of x2 less than 1, as the contrary example, in the acting interval length x2 of the electro-optical effect, the voltage v2 satisfying the acting amount q2(f)=q(f) can be also determined, based on the electro-optical effect acting interval length (0, x2) as shown in FIG. 3. In this case, the normalized voltage v2 becomes higher than the voltage xe2x80x9c1xe2x80x9d.
Also in the present invention according to the above-mentioned invention, the filter may be composed of a plurality of fixed resistance filters connected in a multistage cascade.
Namely, the filter can be composed by directly connecting a plurality of fixed resistance filters where the impedances are matched. Furthermore, it is also possible that the filter composed of the fixed resistance filters is directly connected to the electric waveguide for the impedance matching.
Also in the present invention according to the above-mentioned invention, the filter may be composed of filters connected in a multistage cascade through an amplifier.
Namely, as shown in FIG. 4, filters 32_1-32_3 can be connected in a multistage cascade through amplifiers 31_1 and 31_2 to compose the filter 30. It is to be noted that an amplifier 31_3 is a driver for driving the electric waveguide 20.
Furthermore, in the present invention according to the above-mentioned invention, one or two optical modulators may compose a Mach-Zehnder type optical modulator.
Namely, it is possible to combine the optical modulator 100 of the present invention with a Mach-Zehnder type interferometer to compose a Mach-Zehnder type optical modulator. The Mach-Zehnder type optical modulator 100 may be arranged in one of two optical waveguides or both of the optical waveguides in the Mach-Zehnder type interferometer.